It is soooo important to not look at the alleged discount off either. The same folks that establish the brochure rate and early booking rates establish the sale price.
You should look at the actual price...nothing else. If you want to look at it broken down on a per day or per 7 day cruise basis to compare that makes some sense.
But to book a cruise that is 60% off that is more expensive than one that is 50% off because the discount is allegedly higher isn't usually a smart thing.